Recent changes in U.S. Government policy have left only one source for Keplerian elements, http://www.space-track.org.  Nova has been modified to permit download of Keplerian elements from space-track.org.  However, you must have your own username and password (from space-track) in order to use Nova's download function.

To set up an account, go to space-track.org's "new account" page.  They will send confirmation to you via e-mail.